This invention relates to the ink jet printing technology, and more particularly to an improved method of fabricating a plurality of thermal ink jet printheads from two aligned and bonded substrates which are fastened together by a thermosetting adhesive and a UV curable adhesive inserted into alignment openings formed in portions of the substrates to hold the substrates together until the thermosetting adhesive is cured.
One preferred method of fabricating thermal ink jet printheads is to form the heating elements on the surfaces of one silicon wafer and the channels and small ink supply chamber of reservoir in the surface of another silicon wafer. The two wafers are precisely aligned to insure that the heating elements are aligned to their corresponding channels, and then the two wafers are bonded together. The individual printheads are obtained by dicing the two bonded wafers. This general process has been described in Re. U.S. Pat. No. 32,572 to Hawkins et al. A critical part of this assembly process is the bonding adhesive and its application. Since two silicon wafers are mated that are extremely flat, a thin adhesive coating is sufficient to bond the two together, and a much thicker coat will clog the channels. U.S. Pat. No. 4,678,529 to Drake et al., describes a method of bonding the ink jet printhead components together by coating a flexible substrate with a relatively thin uniform layer of an adhesive having an intermediate non-tacky curing stage. About half of the adhesive layer is transferred from the flexible substrate to the high points or lands of one of the printhead components by placing it in contact therewith, and applying a predetermined temperature and pressure to the flexible substrate prior to peeling it from the printhead component. This causes the adhesive to fail cohesively in the liquid state, assuring that about half of the thickness of the adhesive layer stays with the flexible substrate and is discarded therewith, leaving a very thin uniform layer of adhesive on the printhead component lands. The transferred adhesive layer remaining on the printhead component enters an intermediate non-tacky curing stage to assist in subsequent alignment of the printhead components. The printhead components are aligned and the adhesive layer cured to complete the fabrication of the printhead.
U.S. Pat. No. 4,774,530 to Hawkins discloses an improved ink jet printhead which comprises an upper and lower substrate that are mated and bonded together with a thick film insulative layer sandwiched therebetween. The thick film layer is deposited on the substrate containing the heating elements and addressing electrodes and recesses are patterned in the thick film layer to expose the heating elements to the ink, thus placing them in a pit and to provide a flow path for the ink from the reservoir to the channels by enabling the ink to flow around the closed ends of the channels, thereby eliminating the fabrication steps required to open the channel grooves to the reservoir recess.
FIG. 1 shows a cross-sectional view of an upper channel substrate 2, lower heater substrate 4, and insulative layer 6 of a prior art embodiment equivalent to the '530 patents, whose contents are hereby incorporated by reference. Substrates are shown following an alignment and bonding step. As is disclosed in the '530 patent, several tacking portions 8 are selected around the periphery of the joint substrates, area 8 to be discarded after completion of the tacking step leaving printhead 10 permanently bonded thereto by curing the adhesive applied to the surface of layer 6.
The bonding process includes an initial step of tacking together the substrates. This is accomplished by first forming alignment pits 12 in each predetermined tacking portion 8. The location of these pits is selected so as not to interfere with the functional portion of the printhead 10; e.g., to the right of portion 8. A further etching process creates a plurality of adjacent openings 14 in channel substrate 2. Openings 14 are formed so that they overlie pits 12 when the substrates are aligned. Thus, each portion 8 has a single alignment opening 14 overlying a single pit 12 formed in the insulation layer.
At this point, a thermal setting adhesive 15 is applied to the surface of layer 6, and the substrates are held together in an alignment fixture. This alignment insures that the heating elements formed in the heater substrate underlie channels formed in the channel substrate. Tacking occurs by introducing a UV curable adhesive 20 such as, for example, Loctite 375.TM.. Adhesive 20 is inserted into alignment openings 14 and then downward into pits 12 and irradiated by light from UV source 22. Due to the sloping sides of opening 14 caused by the forming etch process, side portions 20A, 20B of the adhesive are not fully irradiated by the UV light. Thus, adhesive 20 consists of two segments, a cured portion 20C and uncured portions 20A, 20B. When the adhesive is fully cured, the cured adhesive column 20C acts as a tacking "anchor" and improves the sheer strength of the fastened points of the mated wafers. With the cured UV curable adhesive (or cured cyanoacylate) preventing slippage or misalignment between the mated substrates, the substrates are removed from the alignment fixture or IR aligner and moved to a curing oven or vacuum laminator (not shown) and the thermosetting adhesive 15 cured at elevated temperatures. As shown, the unexposed, uncured adhesive segments 20A, 20B initiate a capillary flow radially outward as indicated by arrows 24. The adhesive flow can ultimately be squeezed out the sides of portion 8 and contact bond pads 26 which are formed on the surface of heater substrate 4 causing faulty interconnection to heater energization signals.